Tumors are caused by multiple genetic events, usually involving activation of oncogenes and inactivation of tumor suppressor genes. In humans, but not mice, activation of telomerase reverse transcriptase (TERT) is also observed in ~90% of tumors. Recently, activation of TERT by avian leukosis virus (ALV) integration was observed in B-cell lymphomas in chickens. Since the TERT integrations were clonal, this appears to be an early event in tumor development. In the first aim, this hypothesis will be tested by infection of 10-day chicken embryos with a retrovirus expressing the TERT gene. The rate of tumor induction will be monitored. Retroviral tagging will be employed to identify oncogenes and microRNAs that cooperate with TERT to generate tumors. Cooperativity will be studied in tissue culture systems and in tumor induction. Telomere lengths in the tumors will be analyzed in an attempt to determine whether TERT has roles in addition to telomere lengthening. Finally, telomere expression will be studied at the level of transcription and alternative splicing in the tumors. The hypothesis that retroviruses inactivate tumor suppressors by insertional activation of microRNAs will be tested in the second Aim. miR-155, processed from the non-coding bic gene, is activated in metastatic B-cell lymphomas induced by ALV. Targets of ALV will be identified by bioinformatics, microarray and proteomics analyses and validated with reporter assays. Levels of targets in tumors expressing bic will be determined. The role of individual targets in tumor induction will be studied in tissue culture assays and in vivo. PUBLIC HEALTH RELEVANCE: Since telomerase is involved in most human cancers, it is important to have an animal model system that involves telomerase to further understanding of the disease and development of therapeutics. Chicken B-cell lymphomas induced by retroviral DNA integration into the genome have been shown to involve telomerase and will be studied here. In addition, the role of microRNA-155 in tumor induction will be studied by the identification and characterization of targets important for oncogenesis.